java servlet编写验证码

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Date;
import java.util.Random;
import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ValidateCode
 */
@WebServlet("/ValidateCode")
public class ValidateCode extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ValidateCode() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        // 设置页面不缓存
          response.setHeader("Pragma", "No-cache");
          response.setHeader("Cache-Control", "no-cache");
          response.setDateHeader("Expires", 0);
          // 设置图片的长宽
          int width = 62, height = 22;
          // ////// 创建内存图像
          BufferedImage image = new BufferedImage(width, height,
            BufferedImage.TYPE_INT_RGB);
          // 获取图形上下文
          Graphics g = image.createGraphics();
          // 设定图像背景色(因为是做背景，所以偏淡)
          g.setColor(getRandColor(180, 250));
          g.fillRect(0, 0, width, height);
          // 设置字体
          g.setFont(new Font("Times New Roman", Font.PLAIN, 17));
          // /////设置默认生成4个验证码
          int length = 4;
          java.util.Random rand = new Random(); // 设置随机种子
          /*
           * if (request.getParameter("length") != null) { try { length =
           * Integer.parseInt(request.getParameter("length")); }catch
           * (NumberFormatException e) {} }
           */
          // 设置备选验证码:包括"a-z"和数字"0-9"
          String base = "abcdefghijklmnopqrstuvwxyz0123456789";
          int size = base.length();
          StringBuffer str = new StringBuffer();
          for (int i = 0; i < length; i++) {
           int start = rand.nextInt(size);
           String tmpStr = base.substring(start, start + 1);
           str.append(tmpStr);
           // 生成随机颜色(因为是做前景，所以偏深)
           g.setColor(getRandColor(10, 150));
           // 将此字画到图片上
           // g.drawString(str.toString(), 4, 17);
           g.drawString(tmpStr, 13 * i + 6 + rand.nextInt(5), 14 + rand
             .nextInt(6));
          }
          // 将认证码存入session
          request.getSession().setAttribute("valiCode", str.toString());
          // 图象生效
          g.dispose();
          // 输出图象到页面
          ImageIO.write(image, "JPEG", response.getOutputStream());
         }
         public void doPost(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
          this.doGet(request, response);
         }
         // 给定范围获得一个随机颜色
         Color getRandColor(int fc, int bc) {
          Random random = new Random();
          if (fc > 255)
           fc = 255;
          if (bc > 255)
           bc = 255;
          int r = fc + random.nextInt(bc - fc);
          int g = fc + random.nextInt(bc - fc);
          int b = fc + random.nextInt(bc - fc);
          return new Color(r, g, b);
         }
        }